1. Field of the Invention
The present invention relates to an optical disk apparatus, and more particularly to an optical disk apparatus for optically recording and reading out data to and from tracks on a rotating optical disk by moving an optical head relative to the rotating optical disk.
2. Description of the Related Art
Generally, an image filing system uses an optical disk apparatus. The optical disk apparatus contains an optical disk on which tracks are spirally or coaxially formed. Data is optically recorded and read out to and from the tracks of the optical disk. In the recordinq and reading of the data, the optical disk is rotated and an optical head is moved above and relative to the rotating optical disk.
In a record mode, an original is optically and two dimensionally scanned to read the pictorial information on the original. The optical pictorial information is then converted into corresponding electrical image data. The electrical image data is optically recorded into the tracks of the optical disk by means of an optical head. In a retrieval mode, the recorded data is retrieved by the optical head, and is reproduced in the form of a hard copy or a soft copy.
In the optical head, which is placed close to the surface of the rotating optical disk, a laser beam outputted from a semiconductor laser oscillator is focused on the optical disk by means of an objective lens when data is recorded or reproduced. In the tracking control mode of control of the optical head, the laser beam gathered by the objective traces a track on the optical disk. In the focus control mode, the laser beam is focused on the traced track of the optical disk by means of the objective.
The optical disk apparatus is provided with a linear motor to radially move the optical head across an optical disk in a coarse access mode, and an objective lens drive mechanism for driving the objective lens in a fine access mode.
To access a desired track of the optical disk by a laser beam, the coarse mode is first set up to drive the linear motor and consequently to move the optical head in the radial direction of the optical disk. In this mode, the track is coarsely accessed.
A position of the accessed track on the optical disk is read by the laser beam. If a difference between the read track position and a target track position is small and within a tolerable range, the coarse access mode is switched to the fine access mode. In the fine access mode, the objective is discretely moved toward a desired track by a lens actuator. If the difference is large, the coarse access mode is maintained and the linear motor is driven again to make a rough access to the desired track.
In the coarse access mode, a position detector detects an optical distance scale associated with the optical head, thereby to obtain the position of the optical head and hence the distance that the optical head has moved. A magnetic sensor installed at the linear motor checks the moving direction of the optical head.
In the fine access mode, the number of the tracks crossed by the laser beam collected by the objective lens is counted. By using the count, a moving distance of the objective lens is obtained. The moving direction of the objective lens is checked by the value of a current applied to the lens actuator.
In the optical disk apparatus thus arranged and operated, an eccentricity essentially exists in the optical disk mechanism. The eccentricity creates problems in the access operations. In the coarse access mode, when the optical head reaches a location just before the accessed position and is at a low speed, the eccentricity possibly degrades an accuracy of the access. In the fine access mode, the eccentricity frequently leads to a mistaken recognition of the moving direction as a reverse direction.